SENSITIZATION OF THE TAIL-FLICK REFLEX FOLLOWING EXPOSURE TO EITHER ASINGLE PROLONGED TEST TRIAL OR BEHAVIORAL-TESTING UNDER THE ANALGESICINFLUENCE OF MORPHINE

Although the tail-flick response to radiant heat is widely used in noc iceptive research, there are indications that this benchmark test poss esses some undesirable characteristics. Of present concern is the poss ibility that the supra-threshold stimuli associated with behavioral te sting while under the influence of an effective hypoalgesic manipulati on can alter subsequent tail-flick responses. To examine the effects o f supra-threshold heating of the tail, we exposed anesthetized rats to either (1) manual restraint of the tail during a single tail-flick tr ial to a 5- or 7-sec cut-off, or (2) testing while under the analgesic effects of morphine (5 mg/kg/ml). A single prolonged trial produced h yperalgesia which lasted for 30 min. Following naltrexone injection, h yperalgesia was also found in animals that had been tested while under morphine analgesia. In contrast, animals that received morphine but w ere not tested under its influence did not exhibit hyperalgesia of sim ilar magnitude. Analyses of tail temperature data in the second experi ment indicate that these results are not dependent on shifts in tail t emperature. These results suggest that, in anesthetized animals, expos ure to prolonged tail-flick trials can produce hyperalgesia.